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A report in the Rust repository describes an 11-year-old type-system soundness bug that can let code compile despite a reference outliving the value it points to. The report says triggering memory unsafety requires an intentionally malicious Drop implementation; the issue is rated P-high, and the scope of any real-world impact is not established.
A report in the Rust project’s issue tracker describes an 11-year-old type-system soundness bug that can allow a reference to outlive the value it points to. The issue says a proof-of-concept compiles on current nightly and stable Rust releases back to 1.85.0; maintainers characterize it as P-high and say exploiting it requires an intentionally malicious Drop implementation.
The report’s example wraps an optional value in a generic type with a custom Drop implementation. When the wrapper is dropped, that implementation takes the value and stores it in a global collection. The example assigns a reference to a local String to the wrapper, then allows the local string to go out of scope. The program nevertheless compiles, and later reads through the stored reference.
According to the issue, the sample prints a different string that occupies reused memory when run without AddressSanitizer. With the sanitizer enabled, it crashes with a stack-use-after-scope report. These are the outcomes reported for the reproduction, not evidence that ordinary Rust programs or a particular deployed product have been affected.
The issue says the behavior can be reproduced on current nightly and stable releases at least as far back as Rust 1.85.0. It does not provide a complete affected-version range or identify a fix. A Rust contributor writing in the issue says the report is rated P-high and would have been P-critical even if it had been encountered in real-world use; the lower rating reflects its age and the assessment that it is difficult to hit accidentally.
Why Rust’s Safety Guarantee Is at Stake
Rust’s memory-safety model relies on the compiler rejecting code that could use references after the values they point to have gone out of scope. A case where code passes the borrow checker but can produce a dangling reference is a soundness issue: it challenges the guarantee developers rely on when writing safe Rust, even if the triggering pattern is unusual.
The issue’s author says the demonstration needs an intentionally malicious Drop implementation. That limitation matters: the report does not show that the bug is easy to trigger inadvertently, nor does it establish widespread exploitation. Still, malicious or complex library code can be part of a program’s dependency chain, so compiler soundness reports matter to maintainers and users who need to evaluate where trust in a codebase rests.
For now, the direct practical takeaway is limited but relevant: the reported reproduction deserves investigation, while its presence alone does not show that Rust applications have suffered memory corruption. The project’s response and any eventual fix will clarify the risk and what users may need to do.
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A Long-Lived Compiler Soundness Report
The issue describes the bug as 11 years old, but the supplied material does not give the date it was first introduced or explain how it was discovered. The reporter’s expected result was that the program would be rejected; instead, it compiled and produced the reported output. The issue links to online compiler reproductions, including a version run with AddressSanitizer.
In a note on the issue, contributor lcnr says the Rust type system is intended to prevent memory unsafety and that the Types Team treats soundness work as a priority. The note points to ongoing efforts including a next-generation trait solver, a region-constraint rework and Project Zero. It frames those efforts as part of fixing known type-system soundness problems and reducing the risk of unknown ones—not as proof that this specific issue is already fixed.
“A bug like this which has not been found during the last 11 years cannot actually be that easy to hit.”
— Rust contributor lcnr, in a note on issue #164250
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Affected Versions and Impact Still Need Review
The provided report does not identify the exact release in which the behavior began, list every affected compiler version, or say whether a fix has been developed or released. It also does not quantify how many programs use a pattern that could trigger the issue. The contributor’s view that accidental memory unsafety is very unlikely is an assessment, rather than proof that no affected code exists.
The example shows a way to reach invalid memory through the reported code path, but the material does not establish an attacker’s ability to exploit a specific application, disclose any known incidents, or describe a security advisory. The issue’s eventual triage, root-cause analysis and any mitigation guidance remain relevant to understanding the practical consequences.
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Maintainers’ Triage and Any Fix
The next concrete developments to watch for are updates to Rust issue #164250: confirmation of the affected version range, a technical explanation of the borrow-checker or type-system failure, and details of a proposed correction. If a fix is prepared, release notes or an official advisory would clarify which compiler versions users should update to and whether any additional action is recommended.
Until those details appear in the source material, it is not possible to say when a fix will ship or whether the Rust project will issue user guidance beyond the issue discussion. The report establishes a reproducible concern and the maintainers’ stated priority; it does not yet establish a release schedule or a measured level of real-world exposure.
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Key Questions
What is the reported Rust bug?
The issue describes code that compiles even though a customDrop implementation stores a reference after the value it points to has gone out of scope. The report says this can result in a stack-use-after-scope.Which Rust versions does the report say are affected?
The issue says the behavior occurs on current nightly and stable releases at least as far back as Rust 1.85.0. It does not give a complete affected-version range.Does the report show that Rust programs have been exploited?
No. The supplied material describes a proof-of-concept and a contributor’s view that accidental memory unsafety is very unlikely. It does not report known incidents or establish exploitation of real applications.Why does the issue require a malicious Drop implementation?
In the demonstration, the custom destructor takes a reference held by a wrapper and stores it beyond the referenced value’s lifetime. The contributor says exploitation requires an intentionally malicious implementation of this kind.Has Rust released a fix?
The supplied issue text does not identify a fix or release schedule. The issue’s subsequent updates would be needed to confirm any correction or user guidance.Source: hn
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